The presence of sulphur compounds, such as sulphide, in waste water has many adverse consequences, such as:
corrosive action on concrete and steel, PA1 high oxygen demand (COD), leading to oxygen depletion in the receiving water after discharge of the waste water, involving environmental polution and/or high environmental levies, PA1 toxic effects on man and animals, PA1 serious stench. PA1 much less oxygen, and thus less energy is required, PA1 the process proceeds much faster, PA1 less biological sludge is produced, PA1 no sulphate or thiosulphate is discharged, PA1 there is the possibility of reusing the sulphur. PA1 a: decrease of the activity of the waste treatment plant resulting in a lower purification performance; PA1 b: increase of levies as a result of the increase of the COD load by the washed-out sludge.
While sulphide can be removed from waste water by chemical oxidation, stripping and precipitation, biological purification methods have become increasingly important. Biological removal of sulphide can be performed using phototrophic sulphur bacteria (also accompanied by sulphur production) as well as using denitrifying bacteria. Sulphide can also be converted to sulphate by oxygen consuming bacteria in activated sludge. Sulphur production using oxygen consuming bacteria has advantages over the use of phototrophic bacteria since aerobic conversion proceeds much faster than anaerobic (phototrophic) conversion and light supply in a turbid sulphur reactor is not easy, whereas oxygen can be supplied in an aerobic reactor in a simple way without problems. Nitrate is necessary in the case of denitrifying bacteria.
Advantages of conversion of sulphide into sulphur rather than sulphate include:
A process for the removal of sulphide from waste water by oxidation of the sulphide to elemental sulphur is known from Dutch patent application 8801009 according to which the production of sulphur can be promoted by using a lower oxygen supply than the stoichiometric amount that is needed for sulphate formation. Although a substantial amount of sulphur is produced using this known process, there is a need for improvement of this production, in order to minimize the discharge of soluble sulphur compounds such as sulphide and sulphate.
Another problem connected with biological waste water systems is that sulphide adversely effects the purification efficiency and the sludge retention during aerobic purification of waste water based on a process wherein activated sludge is used. One of the reasons is that sulphide oxidizing, filamentous bacteria such as those of the genera Thiothrix and Beggiatoa can develop in the treatment plants. These filamentous bacteria hamper an efficient settlement of sludge, causing sludge to wash out (bulking out). This has two undesired consequences:
The presence of a high amounts of other sulphur compounds, for example a sulphur content of more than 350-500 mg S/l, or a sulphur related chemical oxygen demand (COD/S) of less than 10, in waste water also causes difficulties in the anaerobic treatment of the waste water, since the sulphide that is formed inhibits the methane producing bacteria. However, anaerobic purification of waste water generally has advantages over aerobic purification: low energy consumption, little sludge increase, methane production, etc. Thus, there is great need for a process allowing anaerobic purification of organic waste effluents, even when these contain high amounts of sulphur compounds.
A process for the anaerobic treatment of waste water containing sulphur compounds is known from European patent application 0.241.999, according to which waste water containing sulphate is purified anaerobically whereby sulphate is reduced to sulphide. The sulphide is then removed from the waste water in the form of hydrogen sulphide (H.sub.2 S). This process has drawbacks in that measures have to be taken (pH adjustment) to ensure that the sulphide is removed from the water to a sufficient extent, and in that the hydrogen sulphide must be subsequently separated from methane and any other gasses, which in turn leads to an effluent which cannot easily be utilized. Furthermore, intoxication of the anaerobic bacteria occurs as well, if the sulphide load is high.
It is generally known that the presence of heavy metals, even in a very low concentration, is undesirable because of the high toxicity for man, plants and animals. Conventional disposal methods, such as hydroxide formation and separation, reversed osmosis and ion exchange, are complicated or do not have the desired result.
A process of removing sulphur compounds and heavy metal ions from water is known from International patent application WO 80/02281. According to that process, sulphate reducing bacteria are cultured in fermentors in the presence of a nutrient solution and a part of the waste water to be treated, and the aqueous sulphide solution which is produced is fed to a precipitation tank together with the remaining bulk of the waste water. The metal sulphides settle in the form of a flocculent precipitate in the precipitation tank, in particular when the waste contains iron (III) ions. Pb, Hg, Cd, Fe, Cu, Ni, Zn, Co, Mn and Ag are mentioned among the metals that can be precipitated. However, this known process does not result in complete sulphate and/or sulphide removal.